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Performance Analysis of Fast Current Loop (FCL) in Servo Drives

This technical brief analyses the functional behavior of the servo loops using fast current loop algorithms in terms of bandwidth and phase margin.

Feb 22, 2019

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Introduction

High performance motor drives in servo control applications are expected to provide high precision and high bandwidth control of current, speed and position loops for superior control of end applications such as robotic arm, CNC machines, and so forth. Since the current loop makes up the inner most control loop, it must have a high bandwidth to enable the outer speed or position loops to be faster. Hence, a high bandwidth FCL is needed in high performance industrial servo control applications. However, the delay due to ADC conversion and algorithm execution limit the current controller bandwidth to about a tenth of the sampling frequency.

The major challenge in digital motor control systems is the influence of the sample and hold (S/H) and transportation delay inside the loop that slows down the system. In a time critical algorithm such as the fast current loop, the latency between feedback sampling and PWM update should be as small as possible. A minimal current loop time not only helps to improve the control bandwidth, but it also enables a higher modulation index (M-I) for the inverter. A higher M-I translates into the higher phase voltage that the inverter can apply on the motor. Higher loop latency will reduce the maximum available voltage and can restrict the rate of current change in the motor, thereby, adversely impacting the controller performance.

In addition to latency considerations, the flexibility to interface various position encoders to the control system can justify the need for FPGAs and external ADCs to implement the fast current loop. However,with the advent of new generation C2000 microcontrollers, it is now possible to replace the functionality of FPGAs and external ADCs. In addition, the MCU can also run speed and position loops with a minimal board space, thereby providing a cost effective solution. This paper outlines the implementation of fast current loop on a C2000 platform running two mechanically coupled motors, and verifies the frequency response of the control loops using TI’s Software Frequency Response Analyzer (SFRA) software library. Dynamic frequency response analysis in real-time on a motor drive system is unique among MCU suppliers and is currently capable only on C2000 MCUs.